Rotary valve and rotary bed technology provides an innovative platform to further improve conventional beaded and structured adsorbent based pressure swing adsorption (PSA) separation processes. For conventional beaded processes, the new technology plays an important role in enhancing the reliability and reducing the foot-print of a PSA plant. For structured adsorbent processes, the technology should aim to reduce the deleterious effects of mass transfer resistance and flow friction pressure drop whilst simultaneously maximizing specific productivity and recovery and eliminating risk of fluidization.
PSA processes typically operate with multiple beds and numerous cycle steps in which each bed undergoes a cyclic sequence of feed, equalization depressurization, blowdown, purge, equalization re-pressurization, and re-pressurization with product and/or feed steps. By way of example, a 9-bed multi-step PSA cycle schedule is presented in FIGS. 1A and 1B (referred to herein collectively as “FIG. 1”) in which F (F1, F2, F3 and F4) indicates a feed step (also referred to herein as the adsorption step), EQD1 indicates an equalization depressurization step, DEQD2 indicates a dual equalization depressurization step, DEQD3 indicates a further dual equalization depressurization step, CnD (CnD1, CnD2 and CnD3) indicates a counter-current depressurization step (also referred to herein as a counter-current blowdown step), PU (PU1, PU2, PU3 and PU4) indicates a product purge step, DEQR3 indicates a dual equalization re-pressurization step, DEQR2 indicates a further dual equalization re-pressurization step, EQR1 indicates an equalization re-pressurization step, and RP/F5 indicates a re-pressurization with product and feed step. FIG. 1A shows the sequence of steps involved in one full PSA cycle and the gas flows into, out of or through a given bed in each step (for example, a given bed undergoes a feed step (F1 to F4) during the first four segment of times, then equalization depressurization step EQD1 during fifth segment of time with another bed that is undergoing equalization re-pressurization step EQR1, and so on). FIG. 1B is a table showing which step each adsorption bed is undergoing at each stage of the process, each row of the bed representing one of the 9 adsorption beds are placed along the vertical direction, and each column representing a segment of time of the cycle (wherein in the first segment of time the first bed as listed in the table is undergoing a first part, F1, of the feed step and the last bed is undergoing step EQR1, in the second segment of time the first bed is undergoing a second part, F2, of the feed step, and the last bed is undergoing step re-pressurization step RP/F5, and so on). Therefore, a row in the table in FIG. 1B represents all the cycle steps a bed undergoes over the entire cycle period and a column represents which cycle steps are being run by which bed at a particular time.
As is evident from the cycle schedule shown in FIG. 1B, in a multi-bed multi-step PSA process, more than one bed may be undergoing a particular step at the same time. For example, in the first segment of time the first and second beds are both undergoing part of their feed step (F1 and F3, respectively) at the same time.
As is also evident from the cycle schedule shown in FIG. 1B, at any particular point in time one or more beds may be undergoing a counter-current blowdown step (CnD1, CnD2, and CnD3) and one or more other beds may be undergoing a counter-current purge step (PU1, PU2, PU3, and PU4). In the blowdown step, a bed that is in the process of being regenerated is depressurized to a waste stream. This is followed by one or more purge steps, where contaminants are flushed from the bed using a low-pressure flow of product. During the blowdown step, flow from the bed being depressurized can be high, especially at the beginning, when bed pressure is the highest. By contrast, the purge step must be performed at a relatively low pressure to be most effective.
In a conventional PSA process, the blowdown and purge effluents are typically combined into the same exhaust stream. In the case of N2 PSA, this is a waste stream, which is vented to the atmosphere. In PSA applications such as H2 purification or biogas upgrading, the exhaust stream can be valuable and recycled as fuel, in which case a surge tank is normally installed to receive and to mix exhaust streams from all steps, including blowdown, bed to bed purge and product purge steps. For H2 PSA, the flow and pressure variation to the exhaust surge tank should be minimized to reduce the variation of Wobbe Index of the exhaust fuel.
In a rotary-bed PSA process the adsorption beds are located in a rotor assembly that is positioned between first and second stator assemblies, each bed having a rotor port at either end of the bed via which gas enters or exits the bed, the rotor assembly being rotated relative to the first and second stator assemblies in order to change the operating modes of the beds (i.e. to change which step of the PSA cycle is taking place in the beds). More specifically, the first and second stator assemblies each include a stator plate having a plurality of slots therein for directing flow of gas to and from the adsorption beds, and rotation of the rotor assembly changes which rotor ports are in alignment with which slots in the stator plates in order to change the operating modes of the beds. Thus, the stator plates and slots therein provide switching valve action for switching the operating modes of the adsorption beds. In order to minimize the number of slots that are needed in the stator plates, it is also conventional for the stator assembly that has the exhaust port or ports for removing the blowdown and purge effluents (i.e. the blowdown and purge exhaust gas streams) to have a stator plate that has a single slot, also referred to herein as an exhaust slot, for receiving all of the exhaust gas streams from all of the beds that are undergoing blowdown or purge steps at the same time. The blowdown and purge effluents can then be combined, as discussed above, in said slot into a single exhaust stream that is directed to the exhaust port of the apparatus.
U.S. Pat. No. 8,470,395 discloses a multi-bed rapid kinetic PSA apparatus utilizing a parallel passage structured adsorbent comprising of a laminated adsorbent sheet (SAPO-34) to separate carbon dioxide (CO2) from a gas mixture comprising methane (CH4). Stator plates are employed to open and close the adsorbent beds to feed and exhaust the process gases. A 28-bed multi-step PSA cycle consisting of production, equalization, reflux (at ambient and sub-ambient pressures), evacuation, product purge and re-pressurization (with feed and product) steps is used to evaluate process performance indicators. The reflux steps are incorporated to enhance the recovery of methane (CH4). In one embodiment multiple exhaust ports are used to reduce pressure drop through those ports by dividing the flow and reducing the gas velocity.
Another example of such a rotary bed process is given in WO 2008/089564, which focuses on technology for sealing valve surfaces including stator plate surfaces of a rotary PSA apparatus. Disclosed therein is a design using two exhaust ports for collecting exhaust streams at different pressure levels, so that if an exhaust compressor is utilized to further process the exhaust gas the size of the compressor can be significantly reduced.